Just so happens I looked up guys on athlinks I ran in college with who were so good (to me at the time) and today (10 years later) they are racing 25min 5k, when 10 years ago on my team they were racing 15min 5k’s even on a bad day. They could run this after weeks of beer pong in our dorms.
Obviously priorities change and some people just don’t find it fun to train anymore. Hiring a coach and/or joining a USATF based club and traveling to train. Plus you need to stay injury free and outside the college environment do not have access to NCAA trainers. A lot of things build up and while these guys never formally quit, they usually train sort of in a halfway in it kind of way and might pop a sub 20min 5k 10 years out